Mechanical abrasion of rail/wheel and brakes and from the catenary (overhead line equipment).

Resuspension of material caused by air turbulence in the stations and tunnels.

PM emitted during night-time maintenance works, including use of traction fuel oil engines, construction works and welding dust.

Cleaning activities.

Surface air uptake from the surface, usually highly polluted by urban (mostly traffic) emissions.

Sporadic incidents, such as flooding of tunnels with high sediment waters, and fires.

Very few published studies have focussed on how PM levels in metro systems might be reduced. A notable exception is the work performed by Johansson and Johansson (2003) who evaluated the effect of washing railways and walls in tunnels on abatement of PM levels in the Stockholm underground, identifying a decrease of around 13% of the PM2.5 levels in the platforms. More recently, Salma et al. (2007) attributed the relatively low PM levels in Budapest metro to the effect of tunnel washing (twice in a year). Similarly, Branis (2006) also measured low levels of PM10 in the Prague underground immediately after a complete clean up and reconstruction. Very recently, Kim et al. (2012) showed that the installation of platform screen doors in a Seoul subway station reduced mean PM10 and PM2.5 levels by around 15%. Similar platform screen door systems have already been installed in a number of metro systems of US, Canada, Brazil, Japan, Denmark, UK, among others, mainly to reduce the number of accidents, but also for more effective temperature and ventilation controls on the platform, having an enormous impact on PM loading breathed on platforms (Querol et al., 2012; Moreno et al., 2014; Martins et al., 2015).